Process for the removal of water from evacuated chambers or from gases

ABSTRACT

A process for the conversion of boric acid to dry boron oxide by thermal decomposition is described. Boron oxide may be produced as a powder or in the form of pellets, and in either form may additionally be enclosed in a gas permeable container to control particulate contamination. Applications are further disclosed for the use of boron oxide formed by this process to remove water from evacuated chambers and from gases. Specific applications include removing water from both inert gases and reactive gases, especially halogen and halogenated gases. Further applications directed to optical amplifiers and gas purifiers are also discussed.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to the field of desiccation andmore specifically to a process for the removal of water from evacuatedchambers or from gases by means of boron oxide obtained by boric aciddecomposition under vacuum or under a dry gas flow.

[0002] Water is one of the main contaminants in vacuum systems and ingases for advanced applications, such as those used in the semiconductorindustry. Consequently, numerous industrial applications call for theremoval of water and water vapor. Water vapor needs to be removed fromevacuated spaces employed as thermal insulation, such as the vitreous ormetallic gaps in thermos flasks and the evacuated panels filled withpolymeric materials used in refrigeration systems. The use of gassorbing materials inside such panels is disclosed, for example, in U.S.Pat. No. 5,544,490. Another application for water sorption includes themanufacture of mechanical microdevices, sometimes referred to asmicromachines or MEMs. A further example of the need to remove water isin polymer-encapsulated integrated circuits as described in U.S. Pat.No. 4,768,081.

[0003] Another important application is in laser devices, for examplepower lasers used in amplifiers for optical fiber communications(hereinafter referred to as “optical amplifiers”). Optical amplifiersconsist, in large part, of a lasing source in an enclosed chamber filledwith an inert gas, typically nitrogen. Upon their manufacture, opticalamplifier chambers frequently contain hydrocarbon impurities as a resultof the production process. These impurities tend, over time, to lowerthe efficiency of the device by forming an obscuring deposit on thelaser's exit window. In order to eliminate these impurities, smallamounts of oxygen are added to the nitrogen atmosphere. The laser beamcauses the oxygen to react with the hydrocarbons to form water and CO₂.The CO₂ does not interfere with the act or operation of the opticalamplifier, however the water has to be removed. The use of impuritygetters in laser enclosures is disclosed in European Patent ApplicationEP707360 A1 published Apr. 17, 1996 and issued as EP707360 B1 on Mar. 4,1998.

[0004] Water removal is also extremely important for purified gases,especially as used in the microelectronics industry for deposition andetching act or operations. The purity levels needed for these processgases continue to increase as the tolerances for defects continue todecrease. For example, the industry currently requires noble gases suchas helium and argon to contain no more than about 5 parts per billion(ppb) of total impurities. The presence of water vapor is particularlyserious in halogen and halogenated gases such as chlorine, hydrogenfluoride, hydrogen chloride, hydrogen bromide, silicon tetrachloride,trichlorosilane and dichlorosilane. Traces of water in these gases,widely used in microelectronics industry, form highly corrosivecompounds such as hydrofluoric acid inside gas pipelines and reactionchambers. Corrosive processes create particles in these ultracleanenvironments lowering yields and necessitating costly downtime andequipment replacement. Other gases employed in the industry, from whichwater needs to be eliminated, include, among others, boron compoundssuch as boron trichloride, boron trifluoride, and diborane; nitrogencompounds such as nitrogen trifluoride, nitrous oxide, nitric oxide, andnitrogen dioxide; hydrides such as silane, arsine, phosphine; sulfurhexafluoride and tungsten hexafluoride; chlorine trifluoride; hydrazineand dimethyl hydrazine.

[0005] Water removal from vacuum chambers and process gases typically iscarried out by means of chemical or physical sorbents. Examples ofphysical sorbents include zeolites, porous alumina, and silica gel.These sorbents are not suitable for many high technology applications,however, because their sorption of water, as well as of the other gases,is reversible, and the sorbed gases may be released in the presence of ahigh vacuum or upon heating. Another problem occurs when the process gasitself is sorbed, for example, when certain zeolites are used to removewater vapor from gaseous HCl, as sorbed HCl diminishes the sorptionefficiency for water.

[0006] Chemical sorbents have been known for a long time. The mosteffective chemical sorbents have been found to be alkaline-earth metaloxides, particularly barium and calcium oxides, and perchlorates ofmagnesium and barium. Other strong chemical sorbents include coppersulfate, calcium and zinc chlorides, and phosphorus pentoxide. Some ofthese materials, however, are not suitable in particular applications.For example, alkaline-earth metal oxides are basic and cannot be usedfor removing water from halogen or halogenated gases because theychemically react with these gases.

[0007] A third class of materials suitable for chemical moisturesorption include zirconium- and titanium-based alloys, generally knownas non-evaporable getter alloys. These alloys sorb a wide range ofgases, including O₂, CO, CO₂, and water. Unfortunately, the sorbingcapacity of these alloys at room temperature is very limited.Additionally, these materials cannot be used to purify reactive gases,such as the above-mentioned halogen and halogenated gases, as they reactwith these gases to form metal halides which then contaminate theprocess gas.

[0008] The problem of water removal from halogen or halogenated gaseshas prompted the development of new materials. U.S. Pat. Nos. 4,853,148and 4,925,646 disclose water removal from HF, HCl, HBr and HI by meansof supported metal halides having the general formula MX_(y), where X isa halogen element and y corresponds to the valency of the metal M, whichmay be 1, 2, or 3. Additionally, these patents disclose metal halides ofthe form MX_(y−1) that may be covalently bonded to a support. U.S. Pat.No. 4,867,960 discloses the use of SiCl₄ and chlorides of metals withvalencies of at least four, with or without support, for water removalfrom HCl. Finally, U.S. Pat. No. 5,057,242 discloses the removal ofwater from chlorosilane gases by using materials of the general formulaR_(a−x),MCl_(x), where R is an alkyl, x is in the range of 0 to a, and Mis a metal selected from the group consisting of the alkali metals,alkaline-earth metals, and aluminum.

SUMMARY OF THE INVENTION

[0009] The present invention provides a process for removing water fromvacuum chambers and gases. In one aspect of the present invention, theprocess includes the following act or operations: first producing boronoxide by boric acid decomposition at a temperature in the range fromabout 70° C. to about 200° C. in a reaction chamber under a dry gas flowor at a pressure lower than about 500 mbar; and secondly, contacting theresulting boron oxide with the vacuum chamber or with the gas from whichwater is sought to be removed.

[0010] Embodiments of the present invention include starting with boricacid either in the form of orthoboric acid, metaboric acid, or anymixture of the two. Further embodiments include using a dry gas flowduring the decomposition reaction where the dry gas is selected from thegroup consisting of noble gases, nitrogen, air, or any mixture of thesegases.

[0011] Other embodiments of the present invention are directed tostarting with boric acid in the form of pellets or loose powder. In thecase of a loose powder, additional embodiments are directed to thearrangement of the powder in beds no thicker than about 1 cm, where morethan one bed may be stacked inside the reaction chamber. Furtherembodiments are directed to the arrangement of the powder in fluidizedbeds, so as to continuously agitate powder in the reaction chamber.Still other embodiments are directed to performing the decompositionreaction on boric acid, either as pellets or loose powder, alreadysealed in a gas permeable container. Further embodiments relate to theconstruction of a gas permeable container from stainless steel havingone or more porous septa made of sintered metal powders, where thepurpose of the septa is to completely retaining the boron oxide withinthe container while allowing for the rapid exchange of gases.

[0012] Still other embodiments are directed to the second act oroperation of the process of the present invention. These include usingthe boron oxide produced in the first act or operation to remove waterfrom gases at temperatures below about 120° C. and to remove water fromevacuated chambers at temperatures below about 80° C. Furtherembodiments are directed to the type of gas that is sought to be dried,which may include halogens, halogenated compounds, and mixtures of thesegases. Another embodiment includes the application of the process of thepresent invention to the removal of water from the evacuated spaceswithin optical amplifiers.

[0013] Yet other embodiments are directed to gas purifiers and opticalamplifiers. Embodiments directed to gas purifiers include gas permeablecontainers filled with boron oxide produced by the decomposition ofboric acid and having porous septa for preventing particulatecontamination while allowing for the exchange of gases. A furtherembodiment is a process for the use of a gas purifier according to thepresent invention to remove water from a gas. Embodiments directed tooptical amplifiers include the incorporation of boron oxide produced bythe decomposition of boric acid, which may or may not be confined withina gas permeable container, within the evacuated space of an opticalamplifier.

[0014] Advantages of the present invention will become apparent to thoseskilled in the art upon a reading of the following detailed descriptionof the preferred embodiments, the examples, and the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a process diagram illustrating the two operations of thepresent invention;

[0016]FIG. 2 is a plan view of a gas purifier according to the presentinvention;

[0017]FIG. 3 is a plot of weight increase as a function of time for awater sorption test in a vacuum chamber performed according to thepresent invention (Curve 1), and performed according to the prior art(Curve 2);

[0018]FIG. 4 is a Fourier Transform Infrared (FTIR) spectrum of moistHCl gas;

[0019]FIG. 5 is a Fourier Transform Infrared (FTIR) spectrum of HCl gasdried according to the process of the present invention; and

[0020]FIG. 6 is a plot of weight increase as a function of time for awater sorption test in a vacuum chamber performed according to analternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The present invention 10, as shown in the process diagram in FIG.1, comprises a first act or operation 12 of decomposing boric acid toform boron oxide, and a second act or operation 14 of contacting theboron oxide with a gas or vacuum chamber sought to be purged of watervapor. Boron oxide, B₂O₃, is a common commercially available productfrequently employed as a vitrification agent in the production ofcertain glass compositions, but it has not previously been employed forwater sorption in industrial applications. The production of boron oxideby boric acid decomposition proceeds according to the reaction:

2 H₃BO₃→B₂O₃+3 H₂O  (I)

[0022] This reaction is well known in the art, and is described, forexample, in “Inorganic Syntheses”, by W. Conard Fernelius, vol. II,McGraw-Hill, 1946, pages 22-23. This text indicates that thedecomposition temperature for boric acid is 200° C. Although thestarting reagent provided in the above reaction is orthoboric acid,H₃BO₃, an intermediate reaction product, metaboric acid, HBO₂, alsoworks well as the starting reagent in the first act or operation of thepresent invention. Hereinafter, reference to boric acid is meant toencompass both orthoboric and metaboric acid unless otherwise stated.

[0023] In the first act or operation 12 of the present invention 10,varying the temperature within a range of about 70° C. to about 200° C.affects both the rate of the decomposition reaction and the porosity ofthe boron oxide product. The rate of reaction decreases with decreasingtemperature, and at temperatures below about 70° C. the rate of reactionbecomes negligible. On the other hand, boron oxide produced attemperatures above about 200° C. has poor water sorption characteristicsdue to reduced porosity. Therefore, the temperature range of about 70°C. to about 200° C. works well for the first act or operation 12 of thepresent invention 10. The second act or operation 14 of the presentinvention 10 may be carried out at temperatures below about 120° C. Theability of boron oxide to sorb water above about 120° C. decreasessignificantly. However, for embodiments in which the boron oxide ismeant to contact a vacuum chamber, an upper temperature limit of about80° C. may be desirable in order to avoid releasing previously sorbedwater.

[0024] Boric acid decomposition may be accomplished within fixed beds orfluidized beds, according to procedures well known in the chemical arts.In the case of fixed bed operation, the thickness of the initial boricacid bed within the reaction chamber has been found to influence theamount of time necessary to carry the reaction to completion. Thin boricacid beds proceed to completion faster than thicker beds because wateris more easily released from thinner layers. Reaction beds less thanabout 1 cm in thickness work well in the first act or operation 12 ofthe present invention 10. Additionally, increasing the boric acid bedsurface area may also reduce the decomposition process time. It istherefore desirable to arrange the boric acid within the reactionchamber in one or more thin wide beds. More than one boric acid bed maybe provided for within the reaction chamber by a plurality of stackedlayers.

[0025] As an alternative, the boric acid may be processed in fluidizedbeds that continuously agitate the boric acid powder as it is convertedto boron oxide. Fluidized beds may be achieved, for example, bydirecting a gas flow from the bottom of the reaction chamber upwardthrough the boric acid powder. Fluidized conditions may also be achievedby vibrating the bed or with mechanical mixing blades. These and othertechniques are well known in the art. Fluidized beds provide additionaladvantages including reducing the time to completion of thedecomposition reaction and preventing clumping of the loose powder.

[0026] In one embodiment of the first act or operation 12 of the presentinvention 10 the decomposition of boric acid is performed under a drygas flow. The gas flow through the decomposition chamber is preferablysufficient to replace the volume of the chamber at least about 5 timesper minute. Flow rates below this level may not be sufficient toeffectively remove from the reaction chamber the water produced by theboric acid decomposition. Under such conditions a deleterious partialpressure of water is maintained in the chamber that may retard completedecomposition. Many gases may be suitable as the dry gas in this processincluding the noble gases, nitrogen, air, and mixtures thereof.

[0027] According to another embodiment of the present invention 10,reaction (I) may be carried out in the first act or operation 12 atpressures below about 500 mbar. Pressure in the reaction chamber may becontrolled, for example, by a porous septum attached to a vacuum pump.Performing the decomposition at pressures of about 500 mbar or less canbe desirable for reducing the time needed to complete the process. Sincethe decomposition reaction produces water vapor, the reaction chambershould be continuously pumped, for example, with a rotary pump. The useof a porous septum is desirable for preventing boric acid or itsdecomposition products from being drawn into the pumping line. Theporous septum, for example, can be a sintered disk of metal powder.

[0028] The second act or operation 14 of the present invention 10involves contacting the boron oxide produced in the first act oroperation with either a gas or with the internal volume of a vacuumchamber. However, for many of the intended applications of the presentinvention 10, the boron oxide produced in the first act or operation 12must be enclosed in a gas permeable container capable of completelyretaining the boron oxide in order to avoid particle contamination ofthe vacuum chamber or gas. Therefore, yet another embodiment of thepresent invention 10 is to prepare boron oxide according to the firstact or operation 12 directly inside a gas permeable container. Such acontainer may be formed, for example, from a stainless steel enclosurehaving an open end capped by a porous septum. As described previously, aseptum may consist of a sintered disk of steel powder. Such a device isdisclosed, for example, in patent application WO 97/19894, incorporatedherein by reference. Further, since boric acid may be readily formedinto boric acid pellets, still yet another embodiment of the presentinvention 10 is to carry out the decomposition process of the first actor operation 12 on boric acid pellets placed within a gas permeablecontainer.

[0029] Different embodiments may be more advantageous depending uponwhether the boron oxide produced in the first act or operation 12 ismade as pellets or as a loose powder, with or without a gas permeableenclosure. For example, some embodiments of the present invention 10will require rapid water sorption in the second act or operation 14,such as in a process gas stream. A loose powder of boron oxide may beadvantageous for such applications, compared to boron oxide pellets,because a loose powder may be more permeable to a flowing gas andprovide a higher surface area.

[0030] On the other hand, the higher density of boron oxide when formedas pellets may provide the advantage of a greater mass for the samevolume compared to a loose powder. Therefore, pellets may beadvantageous where applications in the second act or operation 14require long-term use or small volumes. For example, when the evacuatedvolume in the second act or operation 14 is a refrigerator jacket orsimilar device, the service life may be on the order of decades, and itmay be desirable to seal the boron oxide inside the jacket and neverhave to replace it. Since maximum sorption capacity is a function of theamount of the boron oxide provided, pellets would be advantageous insuch long-term applications because they provide more total mass pervolume than a loose powder. Similarly, for applications such as opticalamplifiers, both long-term use and limited space requirements may makeboron oxide pellets advantageous over loose powders.

[0031] As previously noted, some applications, such as those involvedwith semiconductor processing, may require the use of gas permeablecontainers to prevent particle contamination. A gas permeable containermay be filled with either a loose powder or at least one pellet. In someembodiments the boron oxide may be produced in the first act oroperation 12 by decomposition of boric acid already within a gaspermeable container, and other embodiments may involve filling a gaspermeable container with boron oxide as part of the second act oroperation 14. Gas permeable containers may be desirable, even inapplications where such containers are not essential, because they maybe more easily handled by automated equipment and may reduce thelikelihood of boron oxide spills in production environments.

[0032] Another embodiment of the present invention 10 is an opticalamplifier produced according to the present inventive process. The boronoxide, produced from boric acid according to the first act or operation12, may be in the form of one or more pellets or as a loose powder, andmay or may not be contained within a gas permeable container. In thesecond act or operation 14 of the present invention 10 the boron oxideof the first act or operation 12 is sealed inside the optical amplifiersuch that it is in contact with the amplifier's internal evacuatedspace. One advantage of an optical amplifier according to thisembodiment is that the sealing process may be performed under vacuum ata temperature of approximately 100° C. This combination of low pressureand elevated temperature can help regenerate the boron oxide if ithappened to sorb any water after the completion of the first act oroperation 12. The advantage of the boron oxide regeneration during thefinal sealing process is that the preceding steps in assembling theoptical amplifier need not be carried out under vacuum.

[0033]FIG. 2 shows a gas purifier 20 produced according to the presentinventive process consisting of a container 22 having an inlet 24 and anoutlet 26 and filled with boron oxide 28. The boron oxide 28, producedfrom boric acid according to the first act or operation 12 of thepresent invention 10, may be in the form of one or more pellets or as aloose powder. The boron oxide 28 may be produced in the first act oroperation 12 from boric acid placed within the container 22.Alternately, the boron oxide 28 may be sealed within the container 22 atthe beginning of the second act or operation 14. For the purposes ofthis embodiment, the container 22 is preferably in the form of acylinder with an inlet 24 and an outlet 26 placed at opposite ends, withboth the inlet 24 and the outlet 26 each being fitted with a porousseptum 30 and a fitting 32 for attaching a gas line. In the second actor operation 14 of the invention 10 the boron oxide 28 of the first actor operation 12 is placed in contact with a gas by attaching the gaspurifier 20 in-line with a flowing gas.

[0034] A gas purifier 20 according to this embodiment can vary in sizefrom only a few cubic centimeters, like gas purifiers of the prior artthat are commonly placed upstream of and next to semiconductorprocessing chambers, to purifiers with volumes on the order of cubicmeters. A gas purifier 20 according to this embodiment can bemanufactured according to procedures well known in the art, for example,by employing stainless steel electropolished on the interior surface forthe container 22. A porous septum 30 is preferably positioned at boththe inlet 24 and outlet 26 of the gas purifier 20, thus preventing boronoxide 28 powder from contaminating the gas line. Great Britain PatentNo. 2,177,079, incorporated herein by reference, is referred to forgeneral details relating to the manufacture of gas purifiers.

[0035] Yet another embodiment of the present invention 10 is directed tothe use of the aforementioned gas purifier 20 to purify a gas flow. Useof this process may provide some advantages with respect to prior artsystems used to remove water from flowing gases. First, use of boronoxide does not require a supporting material, thus reducing thenecessary volume of the purifier for the same weight of active material.Secondly, boron oxide is a very light weight |material, further reducingthe weight of a purifier with the same sorption capacity but made frommaterials used in the prior art. Further, boron oxide effectively sorbswater at room temperature, unlike some purifiers of the prior art thatrequire the use of heaters to be effective. Further still, boron oxideonly sorbs water and is effectively inert otherwise. This is contrary tothe behavior of many other absorbents, such as alkaline-earth oxides,that may react with impurities in the gas stream, even if only presentin trace quantities, to form new and undesirable gas species. Extensivetesting conducted on purifiers of the present invention under differentconditions have shown no by-products in the down-stream gas flowsattributable to chemical reactions between the boron oxide and the inletgases (both carrier gases and impurity species), thus indicating thatgeneration of such by-products, if any, are at levels below thedetection limits of the analytical equipment employed, generally below100 ppb.

[0036] Further advantages of this embodiment may be realized when thegas being purged of water is a halogen gas, a halogenated gas, ormixtures of these. Examples of such gases include fluorine, chlorine,bromine, and iodine gases, boron trichloride, boron trifluoride,nitrogen trifluoride, sulfur hexafluoride, tungsten hexafluoride,chlorine trifluoride, mixtures thereof, and mixtures of any of thesegases with any non-halogenated gas. Materials known in the prior art forsorbing water from this category of gases generally require anactivation or conditioning step prior to use and typically have to beheld in the gas stream on a support medium. The use of a support forthese materials, as previously noted, takes up additional space withoutcontributing to the sorption capacity of the device. Additionally, acommon support material is alumina, which at high gas pressures is knownto react with halogenated gases to produce volatile aluminum trihalidesthat may contaminate the gas flow from the purifier outlet. The problemof aluminum trihalide formation has limited the use of prior artpurifiers to low pressure applications. By contrast to the prior art,the purifier of the present invention does not require an activationstep prior to use, does not need a support medium for the boron oxide,and therefore may be used to sorb water from halogenated gases at highpressures without creating aluminum trihalide contamination.

[0037] The invention will be further illustrated by the followingexamples. These non-limiting examples illustrate some embodimentsintended to teach those skilled in the art how to put the invention intopractice and how to provide the best considered way for carrying out theinvention.

EXAMPLE 1

[0038] This example relates to the preparation of boron oxide bydecomposition of boric acid under vacuum.

[0039] 100 g of powdered H₃BO₃ with a purity of 99.5% (Aldrich, Milan,Cat. No. 23, 646-2), is spread over a surface of about 0.3 m² in a steelcontainer; to create a boric acid bed thickness of about 3 mm. The steelcontainer is placed within a vacuum oven. A rotary pump is used toreduce the pressure in the oven to 6.7×10⁻² mbar, and the followingthermal treatment is performed while the oven is continually pumped:

[0040] heating at 1° C./min from room temperature to 120° C.;

[0041] maintaining the temperature at 120° C. for 6 hours;

[0042] heating at 2° C./min up to 140° C.; and

[0043] maintaining the temperature at 140° C. for 17 hours.

[0044] During this treatment the oven pressure reaches a maximum valueof about 2 mbar due to the release of water from the boric acid. Boronoxide powder produced by this method constitutes sample 1. The same testwas performed in a CAHN thermobalance, model D 200, starting with 50 mgof boric acid. A mass spectrometer, branching from the pumping line, wasconnected to the thermobalance, to demonstrate that only water ismeasurably released during the decomposition of the boric acid. Theweight loss of the sample at the completion of the process was about44.1%, compared to a theoretical value of 43.7% for boric aciddecomposition to boron oxide. The small difference can be ascribed tophysically sorbed water within the starting material. The thermobalancetest confirms that sample 1 is boron oxide substantially free of water.

EXAMPLE 2

[0045] This example relates to the preparation of boron oxide by thedecomposition of boric acid in a dry gas flow.

[0046] The procedure described in example 1 is repeated, differing onlyin that the act or operations are carried with a nitrogen flow throughthe vacuum oven at a rate of 1 standard liter per minute (slpm). Inorder to guarantee the absence of water from the nitrogen flow, thenitrogen gas is first passed over a powder bed of an alloy having weightcomposition Zr 76.5%-Fe 23.5%, maintained at 350° C. This alloy isproduced and sold by SAES Getters S.p.A., Lainate, Italy, under the nameSt 198, and is well known in the field for its capacity to sorboxygenated gases. Boron oxide powder produced by this method constitutessample 2.

EXAMPLE 3

[0047] This example relates to the use of Sample 1 to sorb water undervacuum. 51 mg of boron oxide powder from Sample 1 are loaded into thesample holder of a CAHN D 200 thermobalance to measure the weightincrease as water vapor is introduced into the measuring chamber. Duringthe test, the temperature of the measuring chamber is maintained at aconstant 25° C. A rotary pump and a turbomolecular pump are used incombination to initially lower the pressure in the measuring chamber to10⁻⁵ mbar. Next, the pump is isolated from measurement chamber and watervapor is then introduced into the chamber up to a pressure of 5 mbarthrough a needle valve. The weight increase of the sample is recorded bythe thermobalance. The test results, in terms of weight change (ΔP%) asa function of time (t) in minutes, are presented as Curve 1 in FIG. 3.

EXAMPLE 4

[0048] (Comparative)

[0049] This example relates to water sorption under vacuum by acommercial boron oxide sample.

[0050] For the purpose of comparison, the test in Example 3 is repeatedusing instead 291 mg of B₂O₃ with a purity of 99.98% (Aldrich, Milan,Cat. No. 33, 907-5). The test results are presented as Curve 2 in FIG.3.

EXAMPLE 5

[0051] This example relates to water sorption by Sample 1 in an inertgas atmosphere.

[0052] A gas purifier is assembled from an AISI 304 cylindrical steelcontainer having an internal volume of 15 ml filled with powder fromsample 1. The container has openings at both ends provided with fittingsfor gas lines and sintered steel porous septa for confiningparticulates. Prior to the water sorption test, the purifier is slowlyheated to 140° C. for 10 hours in a dry nitrogen flow to completelydegas the walls of the steel container, and then allowed to cool to roomtemperature.

[0053] The test consists of passing nitrogen containing 7 ppm of watervapor through the purifier at a rate of 0.5 slpm. The gas exiting fromthe purifier outlet is analyzed by a Microdowser™ MD2 moisture analyzer,manufactured and sold by SAES Getters S.p.A., Lainate, Italy, which hasa detection limit of 5 ppb for water. At the initiation of the test theamount of water measured in the outlet gas flow is below the analyzer'sdetection limit. The test is continued until the analyzer first detectswater in the outlet gas flow, indicating that the purifier has lost itsefficiency. This occurs after the purifier has been continuouslyoperated for approximately 640 hours. The water capacity of the purifiermay then be calculated, based on the knowledge of the test parameters,to be about 9 l/l (liters of sorbed water per liter of boron oxide).Extensive testing with a flow rate set at 1 slpm and an input waterconcentration in the range of 7-10 ppm has repeatedly demonstratedpurifier capacities in the range from 9 l/l to 25 l/l.

EXAMPLE 6

[0054] This example relates to water sorption in an inert gas atmosphereby boron oxide prepared as in Example 2.

[0055] The test of example 5 is repeated, with the only difference beingthat the purifier is loaded with powder of Sample 2. The purifiercapacity calculated from this test was approximately 4 l/l.

EXAMPLE 7

[0056] (Comparative)

[0057] This example relates to water sorption in an inert gas atmosphereby a commercial sample of boron oxide.

[0058] The test of example 5 is repeated, with the only difference beingthat the purifier is loaded with 15 ml of B₂O₃ as used in Example 4. Thepurifier capacity calculated from this test was about 0.5 l/l. Tworepetitions of this same test gave capacities of 0.9 and 0.3 l/l.

EXAMPLE 8

[0059] This example relates to water sorption from a HCl gas flow byboron oxide prepared according to Example 1.

[0060] A purifier is prepared as described in Example 5. The purifier isdegassed with a dry nitrogen flow at 140° C. for 10 hours, as furtherdescribed in Example 5, prior to the water sorption test. A 0.2 slpmflow of a 1:1 by volume mixture of HCl gas and nitrogen gas containing15 ppm of water vapor is passed through the purifier. The outlet gas isanalyzed with a Protege' FTIR spectrophotometer (Nicolet, Madison, Wis.,USA) equipped with a model 4Runner gas cell (CIC Photonics, Albuquerque,N. Mex., USA). The detection limit of the analyzer for water in HCl isapproximately 30 ppb.

[0061] Additionally, the gas flowing into the purifier is analyzed bydiverting some of the gas through a secondary line to the FTIRspectrophotometer. FIGS. 4 and 5 show, respectively, the FTIR spectra ofthe inlet and outlet gases. The spectra show the infra-red absorbancecharacteristics of the two gas streams in arbitrary units (a.u.) as afunction of wave number (cm⁻¹). Both spectra show the following: anintense peak (indicated in the graphs as A) due to absorbance by HCl;several peaks due to absorbance by CO₂ (indicated in the graphs as B andC); and a set of peaks (indicated in the graphs as D) believed to be dueto absorbance by CO. The FTIR spectrum shown in FIG. 4, collected fromthe inlet gas flow, further shows two sets of peaks, labeled E and F,due to absorbance by water. These two sets of peaks are absent from theFTIR spectrum in FIG. 5, collected from to the outlet gas flow,demonstrating that the water content in the gas from the purifier outletwas below the instrument's detection limit. The test was stopped whenthe purifier had sorbed approximately 9 l/l of water. At that time nowater detected by the analytic instrument in the gas from the purifieroutlet.

EXAMPLE 9

[0062] This example relates to the preparation of boron oxide bydecomposition of boric acid inside a gas permeable container.

[0063] 51 mg of boric acid employed in Example 1 is loaded into acylindrical container, made with AISI steel 304, having one end closed,an internal diameter of 7.5 mm, and height of 1.5 mm. The open end ofthe container is closed with a porous septum of sintered steel AISI316L, having an average pore size of 1 μm. The resulting gas permeablecontainer is mounted on the sample-holder of a CAHN model D 200thermobalance. The thermobalance chamber is evacuated, leaving aresidual pressure of 10⁻⁴ mbar, and the decomposition of the boric acidis begun by heating the sample from room temperature to 105° C. at arate of 2° C./min, followed by maintaining the temperature at 105° C.for 23 hours. A weight loss of 20.3 mg, corresponding to 39.8%, ismeasured, slightly lower than the theoretical value of 43.7%. Thedifference can be ascribed to a slightly incomplete decomposition ofboric acid, most probably due to the porous septum retarding theemission of water. Still under pumping, the measuring chamber of thethermobalance is allowed to cool to room temperature, at which timewater vapor is let in through a needle valve at a pressure of 5 mbar.The weight increase of the sample at 25° C. is measured. The results ofthis test are provided in FIG. 6, plotted as weight increase in mg as afunction of the time in hours. As can be seen in FIG. 6, after 16 hoursof testing the sample weight has increased by 10.3 mg, approximatelyhalf of the weight of the water lost during the boric aciddecomposition.

[0064] From the analysis of the results of the above reported examplesit can be appreciated that boron oxide obtained by thermal decompositionof boric acid at temperatures in the range of 70° C. to 200° C., eitherunder vacuum or under a dry gas flow, may be effective for sorbing waterto levels lower than a few ppb. This effectiveness may be obtained undervacuum and in contact with both inert and reactive gases. Further, thecomparison between FTIR spectra in FIGS. 2 and 3 demonstrates that a gaspurifier for water sorption according to the present invention does notrelease impurities into the outlet gas.

[0065] While this invention has been described in terms of severalpreferred embodiments, it is contemplated that alternatives,modifications, permutations and equivalents thereof will become apparentto those skilled in the art upon a reading this specification. It istherefore intended that the following claims include all suchalternatives, modifications, permutations and equivalents as fall withinthe true spirit and scope of the present invention.

What is claimed is:
 1. A process for removing water, comprising:producing boron oxide by the decomposition of boric acid in a reactionchamber; and contacting said boron oxide with one of a gas and anevacuated volume.
 2. The process according to claim 1, wherein saidboric acid comprises orthoboric acid.
 3. The process according to claim1, wherein said boric acid comprises metaboric acid.
 4. The processaccording to claim 1, wherein said boric acid decomposition is performedunder a dry gas flow.
 5. The process according to claim 1, wherein saidboric acid decomposition is performed at a pressure below about 500mbar.
 6. The process according to claim 4, wherein said dry gas isselected from the group of noble gases, nitrogen, air, and mixtures ofthese gases.
 7. The process according to claim 4, wherein said dry gasis flowed through said reaction chamber at a rate sufficient to replacethe atmosphere within said reaction chamber at least five times perminute.
 8. The process according to claim 1, wherein said boric aciddecomposition is performed at a temperature of between about 70° C. andabout 200° C.
 9. The process according to claim 1, wherein said boricacid is placed within said reaction chamber in at least one bed, said atleast one bed being stackably arranged within said reaction chamber,each said at least one bed having a thickness of no more than about 1cm.
 10. The process according to claim 1, wherein said boric acid isplaced on a fluidized bed moving continuously through said reactionchamber.
 11. The process according to claim 1, wherein said boric acidis contained within a gas permeable container within said reactionchamber.
 12. The process according to claim 11, wherein said boric acidis in the form of pellets.
 13. The process according to claim 11,wherein said gas permeable container is formed from stainless steel witha septum.
 14. The process according to claim 1, wherein said boron oxideis in the form of pellets.
 15. The process according to claim 1, whereinsaid boron oxide is brought into contact with said gas, said gas beingat a temperature below about 120° C.
 16. The process according to claim1, wherein said boron oxide is brought into contact with said evacuatedchamber, said evacuated chamber being at a temperature below about 80°C.
 17. The process according to claim 15, wherein said gas may beselected from the group consisting of fluorine, chlorine, iodine,bromine, boron trichloride, boron trifluoride, diborane, nitrogentrifluoride, nitrous oxide, nitric oxide, nitrogen dioxide, silane,arsine, phosphine, sulfur hexafluoride, tungsten hexafluoride, chlorinetrifluoride, hydrazine, dimethyl hydrazine, and any gas mixturecontaining one or more of the aforementioned gases.
 18. The processaccording to claim 1, wherein said evacuated chamber is part of anoptical amplifier.
 19. An optical amplifier comprising a laser sourcewithin a sealed chamber, said sealed chamber containing a controlledatmosphere of an inert gas and further containing boron oxide in a gaspermeable container.
 20. A gas purifier for removing water from aflowing gas stream comprising: a container, said container having aninlet and an outlet; boron oxide disposed within said container; and afirst porous septum associated with said inlet and a second porousseptum associated with said outlet.
 21. A process for gas purification,comprising flowing a gas through said gas purifier according to claim20.
 22. The gas purifier according to claim 20, wherein said inlet isprovided with a fitting for attaching a first gas line and said outletis provided with a fitting for attaching a second gas line.